$ kotlinc Welcome to Kotlin version 1.1.2-2 (JRE 1.8.0_131-b11) Type :help for help, :quit for quit >>> val nan = Double.NaN >>> nan == nan false >>> nan === nan false >>> val map = mutableMapOf<Double, String>() >>> map[nan] = "a" >>> map[nan] = "b" >>> println(map) {NaN=b}